Oligosaccharides are commonly prepared by the controlled hydrolytic cleavage of starches. In the production of such oligosaccharides, the glycosidic linkages of the starch molecules are partially hydrolyzed to yield at least one oligosaccharide species, and more typically, a mixture of oligosaccharide species. Each oligosaccharide species in the mixture may be characterized by its degree of polymerization (DP), which refers to the number of saccharide monomer units in the molecule. Each oligosaccharide species also may be characterized by its dextrose equivalent (DE), which generally indicates the proportion of aldehyde, hemiacetal or ketone terminal groups in the molecule, and which is a measure of the reducing sugar content of the oligosaccharide, expressed as a percentage of the total dry substance. The DE value and DP profile for a given oligosaccharide mixture can vary substantially, depending, for example, upon the type of starch precursor used to obtain the mixture and the conditions employed for hydrolysis of the base starch.
Oligosaccharide mixtures prepared by the hydrolytic cleavage of starch typically include at least one malto-oligosaccharide species. Malto-oligosaccharides are characterized as having a saccharide backbone that comprises predominantly 1-4 glycoside linkages. Malto-oligosaccharides having a DE less than 20 are known as maltodextrins, whereas malto-oligosaccharides having a DE of 20 or greater are known as syrup solids.
It is known in the art to reduce malto-oligosaccharides and other starch hydrolyzates by reducing the terminal groups in the malto-oligosaccharide or starch hydrolyzate molecule. Such reduced malto-oligosaccharides and other starch hydrolyzates are useful in a variety of applications, including, for example, sweetening agents and texturing agents in products intended for ingestion by animals or humans. Examples of such products include sweets, chewing gums, syrups, food additives, pharmaceutical agents, and so forth. Typically, starch hydrolyzates have been reduced via enzymatic, catalytic, or chemical methods. For example, U.S. Pat. No. 2,280,975 describes a process for the production of polyhydric alcohols via catalytic reduction of mono- and disaccharides. A more recent patent, U.S. Pat. No. 4,322,569, discloses the reduction of monosaccharides by contacting the monosaccharide with hydrogen in the presence of a nickel catalyst in a catalytic bed.
Known processes for the reduction of and starch hydrolyzates suffer from a number of drawbacks. For example, it is often desired to reduce a malto-oligosaccharide to a DE of zero or essentially zero. Typically, such would be accomplished by substantially completely catalytically hydrogenating the malto-oligosaccharide until the desired DE value is obtained. When malto-oligosaccharides are reduced in accordance with such methods, however, the polysaccharide backbones of the individual species in the mixture may become cleaved, as reported, for example in the aforementioned U.S. Pat. No. 2,280,975 with regard to the reduction reaction disclosed therein. Such cleavage of the polysaccharide backbone will cause the DP of the cleaved species in the malto-oligosaccharide to become lower, and will cause an alteration in the overall DP profile of the malto-oligosaccharide mixture. Such alteration of DP profile may cause certain physical properties of the mixture, such as viscosity, to change, thus potentially requiring alteration of processes in which the mixture is intended for use.
Another problem in the art pertains to the color-fastness of malto-oligosaccharides. Malto-oligosaccharides are typically characterized by having a non-zero DE value. One problem with known malto-oligosaccharides is that solutions thereof may tend to yellow under certain conditions, for example, under conditions of heat, alkaline pH, or traces of nitrogen-containing compounds, thus causing visual degradation of the product in which the malto-oligosaccharide is incorporated or other undesired effects. This tendency towards color formations is indicative of the chemical reactivity of the malto-oligosaccharides under the foregoing conditions, particularly towards nitrogen compounds.
In light of these shortcomings in the art, there exists a need for a method for reducing a malto-oligosaccharide to a DE of essentially zero without altering substantially the DP of the malto-oligosaccharide, and particularly for reducing a mixture of malto-oligosaccharides to a DE of essentially zero without altering substantially the DP profile of the mixture. A further need in the art exists for a malto-oligosaccharide product having an improved resistance to color formation. The general objects of the present invention are to provide a method and a product that overcome the foregoing drawbacks of the prior art.